Dry etching method

ABSTRACT

A dry etching method for etching an aluminum (Al) based layer for effectively combatting the after-corrosion in accordance with three aspects. In the first aspect, while a resist mask and chlorine based gas as known per se are used, S 2  F 2  is used during etching of the barrier metal layer. In this manner, residual chlorine in a carbonaceous polymer as a sidewall protection material or a resist mask is replaced by fluorine, while sulfur yielded from S 2  F 2  under conditions of discharge dissociation is deposited to provide for sidewall protection effects. In the second aspect, a SiO 2  mask and an S 2  Cl 2  etching gas are used. Since the sidewall protection material is solely sulfur yielded from S 2  Cl 2 , it becomes possible to avoid the effects of the residual chlorine. In the third aspect, an neutral Ar beam is irradiated at a suitable stage in the etching process for increasing the resistance of the SiO 2  mask against reducing compounds contained in an etching gas for the layer of the aluminum-based material. By irradiation of the neutral beam, a reduction-resistant layer is produced on the surface of the SiO 2  mask to render it possible to reduce the mask thickness without producing problems such as increased step differences on the wafer surface.

This is a division of application Ser. No. 07/828,743 filed Jan. 31,1992, which issued as U.S. Pat. No. 5,217,570 on Jun. 8, 1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a dry etching method and more particularly toa dry etching method in which after-corrosion in etching a layer of analuminum based material may be prevented effectively and in which maskselectivity in case of using an etching mask formed of a silicon oxide(SiO_(x)) based material may be improved.

2. Description of Related Art

As a metallization material for semiconductor devices, aluminum or analuminum-based material, such as an Al-Si alloy with content of 1 to 2%of Si or an Al-Si-Cu based alloy further containing 0.5 to 1% of Cu, isextensively employed. However, as the junction becomes shallower and thecontact hole size becomes finer in keeping up with the recent tendencytowards higher integration of the semiconductor devices, there is anincreasing risk of malfunction such as destruction or deterioration ofthe junction or increased contact resistance due to Al elution into adiffusion layer or Si segregation from the metallization material in thecontact hole. For this reason, it has become customary to provide abarrier metal layer between the metallization material and a siliconsubstrate for preventing an alloying reaction therebetween or siliconsegregation. This barrier metal layer is usually constituted bytransition metals, transition metal compounds such as nitrides,carbides, oxynitrides or borides of transition metals, refractory metalsilicides, or alloys thereof. The barrier metal may not only be in theform of a single layer, but may exist as a combination of layers ofdifferent kinds of materials.

Meanwhile, in processing the layer of the Al-based material, there ispresented a problem of corrosion produced after the end of dry etching,that is after-corrosion, discussed in detail in, for example, pages 101to 106 of "Semiconductor World", April issue, Published by PressJournal. The following is an outline of the after-corrosion.

Dry etching of the layer of the Al-based material is usually carried outusing chlorine based gases, as exemplified by a gas mixture of BCl₃ andCl₂, as disclosed in JP Patent Publication KOKAI No.59-22374 (1984). Theresult is that AlCl₃ as a reaction product or decomposition products ofthe etching gases inevitably remain in the vicinity of the pattern afterthe end of etching. These products are not only adsorbed to the wafersurface but also occluded within the resist mask. If these chlorinebased reaction products or etching gas decomposition products absorb themoisture in the air to form electrolytic liquid droplets, Al is elutedin these droplets to produce corrosion. Besides, while CCl_(x) polymerformed by the reaction between the resist mask and the chlorine-basedactive species plays an important role as a sidewall protection film forassuring shape anisotropy, Cl derived from this polymer also becomesharmful residual chlorine after etching.

The problem of after-corrosion is felt more keenly since Cu started tobe used as additive in the Al-based interconnection or metallizationbecause CuCl as an etching reaction product is left in the patternsection due to its low vapor pressure and, if water is supplied thereto,a local battery is formed which has Cl⁻ as an electrolyte and Al and Cuas electrodes.

If the above mentioned barrier metal structure, or a structure in whichan amorphous silicon layer or the like is stacked as an anti reflectioncoating on the surface of the Al-based material layer for improvingpatterning accuracy, is used, the after-corrosion tends to be produced.Since the cross-section of the stacked structure of heterogeneousmaterials is exposed to the atmosphere as a result of patterning, Alelution is promoted due to local battery effects on formation of theabove mentioned droplets. On the other hand, the micro gaps on theinterfaces of heterogeneous metals provide sites for chlorine orchlorine compounds to be retained.

The after-corrosion is produced to a more or less extent in case ofusing bromine-based gases as etching gases by the mechanism describedabove. For this reason, chlorine and bromine are termed herein ashalogens. However, fluorine is excluded from the generic term of halogenunless specified to the contrary.

As countermeasures for combatting the after-corrosion, there are known(a) a method of plasma cleaning using fluorocarbon based gases, such asCF₄ or CHF₃, (b) a method of ashing off the resist pattern by an oxygenplasma, referred to hereinafter as resist ashing, and (c) a method ofplasma cleaning by NH₃ gas followed by washing with water. Thesecountermeasures are aimed at eliminating residual halogens. That is, thehalogen compounds are converted into fluorine compounds for eliminationthereof upon vaporization, the resist pattern itself containing a largequantity of the residual halogen is removed for eliminating a halogensource, the halogen compound is converted into inert compounds, such asammonium halides or, concurrently with the above, AlF₃ or Al₂ O₃coatings are formed on the surface of the Al-based metallization layerfor suppressing the after-corrosion.

However, the above mentioned countermeasures, aimed at eliminating theresiduals halogen, are not fully effective to suppress theafter-corrosion effectively.

There has also been made a proposal based on a concept different fromthe above concept of eliminating the residual halogen. According to thisproposal, the wafer surface is coated with a carbonaceous polymer, afterthe end of patterning of the Al-based material layer, using a depositiongas such as CHF₃. This technique enables moisture adsorption to beinhibited by the water-repellent carbonaceous polymer to protract thewaiting time for the subsequent process step.

Although this method is highly effective, if executed appropriately, itis necessary to carry out resist ashing simultaneously if there is lefta larger quantity of halogen. In this case, it becomes difficult tocarry out the process due to contradictory requirements that highertemperatures are suited for ashing and lower temperatures are suited forpolymer deposition.

As a further approach, it has also been proposed to use a mask of aninorganic material instead of the resist mask as described above. As tothe mask of the inorganic material, the JP Patent Publication KOKAINo.60-33367 (1985) discloses a process employing an SiO₂ mask. Althoughit is contemplated herein to achieve a high etching resistance, theprocess is thought to be essentially excellent as countermeasuresagainst the after-corrosion because the SiO₂ mask itself is incapable ofoccluding halogens, while a sidewall protection material such as CCl_(x)polymer as a halogen source may not be produced.

However, in order that the process employing the SiO₂ mask may be usedpractically, it is necessary to overcome the latent problem of theincreased step level difference on the wafer surface.

It is virtually impossible to eliminate the SiO₂ mask after etching ofthe Al-based layer because the underlying Al-based layer is usually aninsulating film composed of SiO₂ -based material and, if the SiO₂ maskis removed, the insulating film is removed simultaneously. Thus the SiO₂mask is left and used as a portion of the interlayer insulating filmcovering the patterned Al -based layer. However, this tends to increasethe step level differences on the wafer surface to render it difficultto achieve desired planarization by the interlayer insulating film.

Although this inconvenience may be obviated to some extent bydiminishing the thickness of the SiO₂ mask, it is difficult to diminishthe SiO₂ mask thickness under the current state of the art. A reducinggas, such as BCl₃, is usually added to the etching gas for the Al-basedlayer for removing a native oxide film on a surface thereof. Since theSiO₂ mask is partially reduced to Si by this reducing gas, the SiO₂ maskis attacked by Cl* and thereby partially removed. The SiO₂ mask needs tobe thick enough to take account of the partial removal.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a dry etching methodfor etching an Al-based layer whereby the effects of the residualhalogens may be overcome more effectively than in the conventionalprocess.

It is another object of the present invention to provide a dry etchingmethod whereby, when an SiO₂ mask is used for overcoming the effects ofresidual halogens, mask selectivity may be improved even under areducing atmosphere to thereby enable the mask thickness to be reduced.

For accomplishing the above objects, the present inventor has set up thefollowing three guidelines.

The first guideline is that, while known chlorine compounds or brominecompounds are used to etch the Al-based layer using the resist materialas an etching mask, as conventionally, the gas composition is switchedwhen etching the barrier metal for removing the resist decompositionproducts for forming another sidewall protection film for protecting thepattern sidewall.

The second guideline is that, when employing an SiO₂ mask, a depositionmaterial which does not become a supply source of residual halogens isgenerated in a gaseous phase for assuring high shape anisotropy.

The third guideline is that the SiO₂ mask is renderedreduction-resistant for enabling the mask to be reduced in thickness.

According to the present invention, a compound composed of S (sulfur)and F (fluorine) as constituent elements is used during etching of thebarrier metal in accordance with the first guideline.

Since the resist mask is used herein, a sidewall protection film,composed of CCl_(x) polymer or CBr_(x) polymer, is generated, asconventionally, during etching of the Al-based layer. Then it becomesnecessary to carry out the next step of removing the sidewall protectionfilm or substituting another element not producing corrosion for theresidual halogen and simultaneously forming a substitutive sidewallprotection film. It is a compound containing S and F as constituentelements which have attracted the present inventor's attention. If theetching of the barrier metal is performed using an etching gas composedmainly of the above compound, CCl_(x) or CBr_(x) may be decomposed andremoved, at least partially, depending on the operating conditions. Evengranting that small amounts of the resist decomposition products oraluminum halides are left, Cl or BF therein are replaced by F. This isalso supported by the fact that the relative magnitudes of theinteratomic bond energy between the atoms concerned are expressed byinequalities C--F>C--X and Al--F>Al--X, where X denoted Cl or BF.Simultaneously, sulfur is deposited on the pattern sidewall, above all,on the sidewall of the barrier metal, so as to play the role of sidewallprotection during etching and pass i vat i on after etching.

Meanwhile, the present Assignee previously proposed a technologyapparently similar to the above mentioned aspect of the presentinvention and which resides in etching a layer of an Al-based material,using an etching gas mainly composed of a compound containing S and Clas component elements and an etching mask formed by a three-layer resistprocess. According to the three-layer resist process, an intermediatelayer composed of an SOG (spin-on-glass) etc. and a lower resist layerpatterned with the use of an intermediate layer as a mask. Thus, resistdecomposition products also are not supplied into the system in thiscase. This prior-art technology, however, is mainly aimed at utilizingsulfur deposition for achieving high anisotropy even in cases whereinthe resist decomposition products can not be utilized for sidewallprotection, while it leaves much to be desired with respect tocombatting the after-corrosion. It is because the risk is high with theabove described prior-art technology that resist residues remain on awafer during patterning of the lower resist layer and, if residualchlorine produced during etching of the Al-based layer is taken up bythese residues, the after-corrosion is produced inevitably.

On the contrary, since no organic material is employed in the etchingmask in accordance with the present invention, the above mentionedproblem is not produced, so that the present invention differs from theabove described prior-art technology in both the operation and effects.

In the present invention in accordance with the second guideline,described above, since the decomposition products of the etching maskcan not be utilized for sidewall protection, the sidewall protectionfilm is produced from products derived from a gas system. Specifically,the present inventor drew attention to compounds containing S and Cl orS and Br as component elements and arrived at a concept of performing alow temperature etching using an etching gas mainly composed of thesecompounds for depositing sulfur on the pattern sidewall. In distinctionfrom the conventional sidewall protection film composed of CCl_(x) orCBr_(x), the sulfur deposits formed in this manner do not become asupply source of Cl or Br. Besides, the sulfur deposit may be sublimedoff easily on heating so that, even though minor amounts of residualhalogens have been taken up in inner pores or the like, these are notretained ultimately. Therefore, the sulfur deposit may be utilized notonly as a sidewall protection film for achieving anisotropy, but alsobecomes a passivation film for protecting the pattern sidewall from theatmosphere from the end of etching until the initiation of thesubsequent process step.

In the present invention in accordance with the third guideline, neutralbeam irradiation is employed.

For example, in Extended Abstract of the 38th Spring Meeting (1991) ofthe Japan Society of Applied Physics and Related Sciences, page 409,lecture number 29p-P-6, a report has been made of the results ofexperiments concerning the irradiation of a neutral beam on an SiO₂layer formed by a thermal oxidation method. It is seen from this reportthat the analyses, which were conducted of the surface SiO₂ compositionby a Rutherford back scattering spectroscopy (RBS) after irradiationindicated that the SiO₂ composition was maintained. It is also seen fromthe report that the analyses of a depth profile by an Auger electronspectroscopy (AES) indicated that, with an unirradiated sample, SiO₂ wasreduced with the progress of the ion etching and a Si absorption peakwas observed, whereas, with a sample irradiated by a neutral beam, no Siabsorption peak was observed. This means that at least the surface layerof the SiO₂ has been rendered reduction-resistant as a result ofirradiation with the neutral beam.

Although the mechanism of the formation of the reduction-resistant layeris not clear at present, the results of the experiment shown by theabove report indicate that irradiation with the neutral beam is highlyeffective in improving reduction resistance of the SiO₂ mask in thecourse of etching of the Al-based layer. That is, if a wafer isirradiated with the neutral beam before or during etching of theAl-based layer to modify the surface of the SiO₂ mask to render itreduction-resistant, there is no risk of lowering of mask selectivityalthough reducing gases are added to the etching gas. Thus it becomespossible to diminish the thickness of the SiO₂ mask.

While the above is the basic concept of neutral beam irradiation, threedifferent methods of approach may be thought of in selecting the timingof etching of the Al-based layer and neutral beam irradiation.

The first approach is to carry out neutral beam irradiation beforeetching of the Al-based layer.

The second approach is to perform the neutral beam irradiation andpartial etching of the Al-based layer along its thickness in analternate manner. Although a reduction-resistant layer produced by theneutral beam irradiation on the surface of the SiO₂ is remarkablyimproved in etching resistance as compared with an unirradiated SiO₂mask, removal of a certain quantity of the material by ion sputteringand some reduction by the reducing gases occur inevitably if the mask isexposed to high density plasma. For this reason, the etching process forthe Al-based layer is divided into plural stages and neutral beamirradiation is again performed after the reduction-resistant layer isdiminished or has disappeared for re-forming a reduction-resistantlayer. Although the number of process steps is increased, a thinner SiO₂mask may be available than is possible with the first approach.

The third approach is to effect neutral beam irradiation and etching ofthe Al-based layer simultaneously. In this case, a reduction-resistantlayer is present at all times on the SiO₂ mask surface and the Al-basedlayer is etched by a so-called neutral beam assisted etching mechanism.

The neutral beam assisted etching is a technique publicized in, forexample, Extended Abstract of the 51st Autumn Meeting (1990) of theJapan Society of Applied Physics, p.483, lecture number 27p-ZF-5 orDigest of Papers, 1990 3rd. MicroProcess Conference, B-5-3, and residesin performing anisotropic etching while a kinetic energy of a neutralbeam is supplied to an etchant adsorbed on the surface of an etchedmaterial. This process has been developed with an eye drawn to the factthat the mechanism of the ion assisted reaction in plasma etchingdepends mainly on the kinetic energy of the ions while the electricalcharges of the ions are intrinsically not necessary. The technique ishighly effective in avoiding radiation damage by charged particles. Thethird approach thus results in a process with ultra-high selectivity andultra-low damage.

It will be seen from the above that the present invention provideseffective and practically useful measures for combatting theafter-corrosion to render it possible to improve the yield andreliability of semiconductor devices significantly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a to 1d are schematic cross-sectional views showing a typicalprocess according to the present invention, step by step, wherein FIG.1a shows the state in which a photoresist pattern as an etching mask hasbeen formed on a multilayer film, including an Al-1%Si layer, FIG. 1bshows the state in which etching of the multilayer film has beencompleted as far as the Al-1%Si layer, FIG. 1c shows the state in whicha TiW layer has been patterned and FIG. 1d shows the state in which thephotoresist pattern and a sidewall protection film are both removed.

FIGS. 2a to 2c are schematic cross-sectional views showing anothertypical process according to the present invention, step by step,wherein FIG. 2a shows the state in which an SOG pattern as an etchingmask has been formed on a multi layer film, including an Al-1%Si layer,FIG. 2b shows the state in which a metallization pattern has been formedby etching and FIG. 2c shows the state in which the SOG pattern and thesidewall protection film have been removed.

FIG. 3 is a schematic cross-sectional view showing a constructionalexample of an etching apparatus employed in practicing the presentinvention.

FIGS. 4a to 4c are schematic cross-sectional view showing a furthertypical process according to the present invention, step by step,wherein FIG. 4a shows the state in which an SiO₂ mask has selectivelybeen formed on an Al-1%Si-0.5%Cu layer stacked on a barrier metal layer,FIG. 4b shows the state in which a reduction-resistant layer has beenformed on the SiO₂ mask by irradiation of a neutral Ar beam, and FIG. 4cshows the state in which the Al-1%Si-0.5% Cu layer and the barrier metallayer have been etched anisotropically.

FIGS. 5a to 5d are schematic cross-sectional view showing a stillfurther typical process according to the present invention, step bystep, where FIG. 5a shows the state in which a reduction-resistant layerhas been formed on a surface of an SiO₂ mask by neutral beamirradiation, FIG. 5b shows the state in which the Al-1%Si-0.5% Cu layerha been etched halfway, FIG. 5c shows the state in which areduction-resistant layer has again been formed on the surface of theSiO₂ mask by neutral AF beam irradiation, and FIG. 5d shows the state inwhich the remaining portion of the Al-1% Si-1% Cu layer as well as thebarrier metal layer has been etched anisotropically.

FIG. 6 is a schematic cross-sectional view showing a constructionalexample of a neutral beam assisted etching apparatus employed forpracticing the present invention.

FIGS. 7a and 7b are schematic cross-sectional view showing a yet furthertypical process according to the present invention, step by step,wherein FIG. 7a shows the state in which an Al-1%Si-0.5%Cu layer hasbeen etched halfway by neutral beam assisted etching, and FIG. 7b showsthe state in which the etching has been terminated.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be explained with reference to certainnon-limitative Examples.

EXAMPLE 1

In the present Example 1, etching as far as an Al-1%Si layer was madeusing a usual resist mask and a chlorine-based gas and etching as far asbarrier metal layer was made using an S₂ F₂ gas. The process will beexplained by referring to FIGS. 1a to 1d.

A wafer was first prepared, in which, as shown in FIG. 1a, a TiW layer 2about 200 nm thick, an Al-1%Si layer 3, about 400 nm thick and anamorphous silicon layer 4 about 30 nm thick, were stacked sequentiallyto form a multilayer film 5, and a photoresist pattern 6 was formed asan etching mask. Meanwhile, the TiW layer 2 was used as a barrier metallayer, and the amorphous silicon layer 4 was used as an antireflectionfilm for improving accuracy in photolithography used for forming thephotoresist pattern.

The amorphous silicon layer 4 and the Al-1%Si layer 3 were then etchedwith the photoresist pattern 6 as a mask. That is, the above wafer wasset on a magnetic microwave plasma etching apparatus and etching wasperformed under the conditions of a BCl₃ flow rate of 20 SCCM, a Cl₂flow rate of 30 SCCM, a gas pressure of 1.3 Pa (10 mTorr), a microwavepower of 1000 W and an RF bias power of 100 W (13.56 MHz). In the courseof the etching process, a carbonaceous polymer CCl_(x), produced fromthe photoresist pattern 6 sputtered by BCl_(x) ⁺ or Cl_(x) ⁺ ions, wasdeposited on the pattern sidewall to form a CCl_(x) based sidewallprotection film 7. In this manner, the amorphous silicon pattern 4a andthe Al-1% Si pattern 3a were formed with good shape anisotropy, as shownin FIG. 1b. It is noted that residual chlorine 8 was adsorbed on oroccluded in the photoresist pattern 6 and the CCl_(x) based sidewallprotection film 7, as shown schematically in FIG. 1. Meanwhile theresidual chlorine herein means reaction products, such as AlCl₃,decomposition products of the etching gas or Cl atoms constituting theCCl_(x) based sidewall protection film 7, exhaustively.

A cooling medium, such as ethanol, was then circulated through a coolingconduit housed in a wafer setting electrode, and the barrier metallayer, that is the TiW layer 2, was etched, under the conditions of anS₂ F₂ flow rate of 50 SCCM, a gas pressure of 1.3 Pa (10 mTorr), amicrowave power of 850 W, an RF bias power of 100 W (13.56 MHz) and awafer temperature of -50° C. While the TiW layer 2 was etched in thecourse of this process by the action of SF_(x) ⁺ S⁺ or F*, decompositionproducts of the photoresist pattern 6 were scarcely supplied. Instead,sulfur yielded in the plasma by discharge dissociation of S₂ F₂ wasdeposited on the pattern sidewall to form a sulfur-based sidewallprotection film 9. As a result, a TiW pattern 2a having a goodanisotropic shape was formed and a metallization pattern 5a exhibitingshape anisotropy as a whole was formed, as shown in FIG. 1c. Also, partof the CCl_(x) was decomposed and removed, while the remaining partthereof underwent Cl-F exchange so as to be converted into a CF_(x)based sidewall protection film 7a. In this manner, the residual chlorine8 on the sidewall section of the metallization pattern 5 a was removedcompletely.

Meanwhile, FIG. 1c shows a typical state of formation of the sidewallprotection film and, as the case may be, the CCl_(x) based sidewallprotection film 7 is removed substantially completely, whereas thepattern sidewall was substantially covered only with a sulfur-basedsidewall protection film 9. Besides, there may be occasions wherein thesurface of the CF_(x) based sidewall protection film 7a is additionallycovered by the sulfur-based sidewall protection film 9.

The wafer was then transferred to a plasma ashing device and, whilst thewafer was heated to about 150° C., resist ashing was performed under theconditions of O₂ flow rate of 50 SCCM, a CH₃ OH flow rate of 30 SCCM, agas pressure of 133 Pa (1 Torr) and a microwave power of 850 W. As aresult, the photoresist pattern 6 and the CF_(x) based sidewallprotection film 7a were decomposed and removed and, concomitantly, theresidual chlorine 8, occluded in the photoresist pattern 6, was alsoremoved. By the heating which occurred at this time, the sulfur-basedsidewall protection film 9 was also removed promptly. In addition, theresidual chlorine adsorbed on the wafer surface was replaced by F*yielded from the fluorine-based gas.

On the wafer processed in the above described manner, the time whichelapsed since the wafer was taken out into open air until the occurrenceof the after-corrosion was prolonged to three or more times of that withthe conventional wafer.

Although the chlorine-based gas was used in the present Example foretching the amorphous silicon 4 and the Al-1%Si layer 3, a Br-based gasmixture, such as BBr₃ --Br₂ gas, may also be employed. Since it sufficesin this case if Cl (chlorine) in the above described process reads Br(bromine), detailed description is omitted for simplicity.

In the present Example, S₂ F₂ was used as a compound containing S and Fas constituent elements. This compound is among the compounds known assulfur fluorides, which also include SF₂, SF₄ and S₂ F₁₀ as stablecompounds. Besides, SF₆ is also among the sulfur fluorides and put topractical use as a gas for dry etching. However, this compound is notconvenient for the purpose of the present invention because it has ahigh F/S ratio (ratio of the number of fluorine atoms to that of sulfuratoms in one molecule) and hence generates a large quantity of F* and,besides, it has been confirmed that the compound hardly yields sulfur ondischarge dissociation.

EXAMPLE 2

In the present Example, a multilayer film including an Al-based layerwas etched using an S₂ Cl₂ gas and an etching mask composed ofspin-on-glass (SOG). This process will be explained by referring toFIGS. 2a to 2c.

A wafer was first prepared, in which, as shown in FIG. 2a, a TiW layer12 about 200 nm thick, an Al-1%Si layer 13 about 400 nm thick and anamorphous silicon layer 14 about 30 nm thick, were stacked sequentiallyto form a multilayer film 15, and an SOG pattern 16 was formed as anetching mask. Meanwhile, the TiW layer 12 was used as a barrier metallayer. The SOG pattern 16 was patterned using a photoresist mask, notshown, while the amorphous silicon layer 14 was provided as anantireflection film for improving accuracy in photolithography whenforming the photoresist mask.

The multilayer film 15 was then patterned, using the SOG pattern as themask 16, Thus the wafer was set on, for example, a wafer settingelectrode of a magnetic microwave plasma etching system. It is notedthat the wafer setting electrode is designed for circulating a suitablecooling medium through a cooling conduit housed therein for maintainingthe wafer temperature at a temperature at or below 0° C. during theetching. The etching was carried out under the conditions of an S₂ F₂flow rate of 50 SCCM, a gas pressure of 1.3 Pa (10 mTorr), a microwavepower of 850 W, an RF bias power of 100 W (13.56 MHz) and a wafertemperature of -30° C. In the course of this etching process, etching ofthe multilayer film 15 proceeded by the action of chlorine-based activespecies yielded by discharge decomposition of S₂ Cl₂. However, sincethis etching naturally proceeded with a high selectivity of anunderlying insulating film 11, selectivity was similarly high as far asthe SOG pattern 16, formed of a silicon oxide based material as is theinterlayer insulating film 11, is concerned. Thus the SOG pattern 16 wasscarcely sputtered under the above mentioned conditions, and adecomposition product, with a low vapor pressure capable of beingdeposited on the pattern sidewall, was not produced. Instead, the sulfurproduced in the plasma due to discharge decomposition of S₂ Cl₂ wasdeposited on the pattern sidewall since the wafer was already cooled, sothat sulfur-based sidewall protection film 17 was formed competitivelywith the etching reaction. As a result thereof, as shown in FIG. 2b,patterns of respective layers, shown by the same reference numerals butwith suffixes a thereto, were formed at the same time that an Al-1%Sipattern 13a was formed, so that a metallization pattern 15a, which hasgenerally a good shape anisotropy, was formed.

It is noted that the sulfur-based sidewall protection film 17 is itselffree from Cl as a constituent element, in a manner different from theconventional sidewall protection film composed of, for example, COl_(x).For this reason, the film 17 does not become a supply source forresidual chlorine, even when deposited on the metallization pattern 15a.Rather, the protection film 17 plays the role of passivating thesidewall for preventing Cl* from being adsorbed by isolating thesidewall from the etching atmosphere.

The wafer was then transferred to a plasma etching device and, whilstthe wafer was heated to about 150° C., the SOG pattern 16 was removedunder the conditions of C₃ F₈ flow rate of 50 SCCM, a gas pressure of 5Pa and a microwave power of 850 W. By the heating which occurred at thistime, the sulfur-based sidewall protection film 17 was also promptlysublimed off. In addition, the residual chlorine adsorbed on the wafersurface was replaced by F* yielded from the fluorine-based gas.

On the wafer processed in the above described manner, the time whichelapsed since the wafer was taken out until the occurrence of theafter-corrosion was prolonged to three or more times of that with theconventional wafer.

In the present Example, S₂ Cl₂ was used as a compound containing S andCl as constituent elements. This compound is among the compounds knownas sulfur chlorides, which also include S₃ Cl₂, Scl₂ etc. as stablecompounds.

The compounds from which similar effects may be expected include S₂ Br₂,S₃ Br₂ and SBr₂ containing S and Br as constituent elements.

Although SOG which could be formed at a lower temperature was used inthe above Example as a constituent material for a silicon oxide basedetching mask, a silicon oxide based thin film which may be formed bye.g., bias sputtering may also be employed.

EXAMPLE 3

In the present Example, a reduction-resistant layer was formed on thesurface of an SiO₂ mask, using a neutral Ar beam, and an Al-1%Si-0.5%Culayer and a barrier metal layer were subsequentially etched using a BCl₃/Cl₂ /HBr mixed gas.

An etching apparatus employed in the present Example is shownschematically in FIG. 3. This system is made up of a neutral beamirradiating unit 100 and an RF bias impressing type magnetic microwaveetching unit 101, interconnected by means of a gate valve 28.

The neutral beam irradiating unit 100 is mainly composed of an ionizingchamber 21 for ionizing a feed gas supplied from the direction shown byarrow A in FIG. 3 by means of a gas supply duct 22, a neutral beamirradiation chamber 24 connected to the ionizing chamber 21 by means ofa multiaperture electrode 23 and housing a wafer stage 26 on which toset a wafer 40, and a charged particle removal electrode 27 disposedabove the wafer 40 in the neutral beam irradiation chamber 24 forremoval of charged particles. The inside of the neutral beam irradiationchamber 24 is connected to a vacuum source, not shown, by means of anair vent duct 25, and i s evacuated to high vacuum in a direction shownby arrow B in FIG. 3.

With the present apparatus, ions produced in the ionizing chamber 21 areintroduced by means of the multiaperture electrode 23 into the neutralbeam irradiation chamber 24 where they are neutralized by a chargeexchange reaction with the background gas and thereby converted into aneutral beam which is then irradiated on the wafer 40 after removal ofunneutralized residual ions by the charged particle removal electrode27.

On the other hand, the magnetic plasma etching unit 101 is mainlycomposed of a magnet ton 29 for generating the microwave of 2.45 GHz, arectangular waveguide 30 and a circular waveguide 31 for guiding themicrowave generated from the magnetron 29, a quartz bell jar 32 fortaking in the microwave and adapted for generating a plasma P₁ in theinside thereof by electron cyclotron resonance (ECR), an etching gassupply duct 34 for supplying an etching gas from the directions shown byarrows C₁, C₂, a solenoid coil 33 mounted for encircling the bell jar 32and adapted for producing a magnetic flux with a magnetic flux densityof 8.75×10⁻² T (=875 Gauss), satisfying the ECR conditions, in asuitable region in the bell jar 32, and a sample chamber 35 connected tothe bell jar 32 and housing a wafer setting electrode 37. The samplechamber 35 may also be advantageously employed in transporting the wafer40. The unit 101 also includes an RF power source 38 for applying an RFbias to the wafer setting electrode 37. The sample chamber 35 isconnected by means of vent duct 36 to the vacuum source, not shown, andis evacuated to high vacuum in the direction shown by arrow D in FIG. 3.A cooling duct 39 is housed within the wafer setting electrode 37 and asuitable cooling medium from a cooling device, such as a chiller, notshown, is circulated in a direction shown by arrows E₁, E₂ for coolingthe wafer 40.

A typical process which will be executed by the above apparatus will beexplained by referring to FIGS. 4a to 4c.

First, as shown in FIG. 4a, a barrier metal layer 44, comprised of a Tilayer 42 about 30 nm thick and a TiON layer about 70 nm, and anAl-1%-0.5%Cu layer 45 about 400 nm thick, were formed by sputtering inthis order on an SiO₂ interlayer insulating film 41, and an SiO₂ mask 46about 50 nm thick, patterned to a predetermined contour, was formed onthe layer 45. It is noted that, for forming the SiO₂ mask 46, an SiO₂deposition layer was formed on the entire wafer surface by CVD under anormal pressure by using, for example, a O₃ -TEOS (tetraethoxysilane)gas, a resist mask was then formed using a novolac based positivephotoresist manufactured by TOKYO OKA KOGYO KK under the trade name ofTSMR-V3 and an g-line stepper, and the SiO₂ deposition layer wasselectively etched within a magnetron reactive ion etching (RIE) deviceusing a C₃ F₈ gas.

The wafer 40 was set on the wafer stage 26 of the neutral beamirradiation unit 100. An Ar gas was introduced by the gas supply duct 22into the ionizing chamber 21 for generating Ar⁺ ions therein. These Ar⁺ions were taken by means of the multiaperture electrode 23 as an ionbeam which was converted by a charge exchange reaction into a neutral Arbeam which was caused to be incident on the wafer 40 by means of thecharged particle removal electrode 27. As a result of the irradiation, areduction-resistant layer 46a was formed on the surface of the SiO₂ mask46, as shown in FIG. 4b.

The wafer 40 was then transported into the sample chamber 35 of themagnetic microwave plasma etching unit 101 by means of the gate valve 28and set on the wafer setting electrode 37. Ethanol was circulated as acooling medium through the cooling conduit 39 for maintaining the waferat a temperature of approximately -30° C. during etching. TheAl-1%Si-0.5% Cu layer 45 and the barrier metal layer 44 were etchedunder the conditions of a BCl₃ flow rate of 20 SCCM a Cl₂ flow rate of30 SCCM, an HBr flow rate of 40 SCCM, a gas pressure of 1.3 Pa (10mTorr), a microwave power of 1000 W and an RF bias power of 30 W (2MHz).

In the course of this etching process, etching proceeded anisotropicallyby a mechanism in which the radical reaction by Cl* and Br* radicalswere assisted by B⁺, BCl⁺, Cl⁺ and Br⁺ ions. In this manner, theconstituent elements of the Al-1%Si-0.5%Cu layer 45 and the barriermetal layer 44 were removed in the form of chlorides or bromides. InFIG. 4c, only AlCl_(x), AlBr_(x) and TiCl_(x) are entered as typicalexamples of these products. As a result thereof, an Al-Si-Cumetallization layer 45a and a barrier metal layer 44a presenting shapeanisotropy were formed. In the drawing, the patterns formed by etchingare indicated by reference numerals of the corresponding unetched layerswith suffix letters a appended thereto.

Meanwhile, the above etching gas was added to by BCl₃ as a reducing gasfor removing a native oxide film existing on the surface of theAl-1%Si-0.5%Cu layer 45. Consequently, with the conventional process inwhich the above etching gas was used, the SiO₂ mask was removed in theform of SiCl_(x), SiBr_(x) or BO_(x), so that the resistivity was of theorder of 7 at most. For this reason, film thickness of the SiO₂ mask ofthe order of 200 nm was necessitated in order to take account ofdeterioration which may be produced by ion impacts. According to thepresent invention, since the reduction-resistant layer 46a was formed onthe surface of the SiO₂ mask 46, selectivity was improved toapproximately 20, such that the SiO₂ mask of only 50 nm thick served thepurpose. Consequently, the subsequent planarization step by theinterlayer insulating film could be achieved easily.

Although the state in which the reduction-resistant layer 46a has beenremoved completely is shown in FIG. 4c as an ultimate state, thereduction-resistant layer 46a may be left under optimized conditionsobtained by suitably adjusting the BCl₃ flow rate or the RF bias.

Also, according to the present invention, the resistance againstafter-corrosion may be improved significantly because the sidewallprotection film or the resist mask ready to occlude residual halogensare not present on the wafer 40. After the end of etching, the wafer 40was allowed to stand in open air in order to check for resistanceagainst after-corrosion. It was found that the after-corrosion occurredin 7 days, as compared to 2 days for the conventional process employingthe resist mask, indicating that the resistance against after-corrosionwas improved significantly.

EXAMPLE 4

In the present Example, a step of forming a reduction-resistant layer onthe surface of an SiO₂ mask using a neutral Ar beam and a subsequentstep of partially etching an Al-1%Si-0.5% Cu layer along the thicknessthereof using a BCl₃ /Cl₂ /HBr mixed gas were repeated twice in thisorder. This process is explained by referring to FIGS. 5a to 5d.Meanwhile, the same numerals are used in FIGS. 5a to 5d to depict theparts shown in FIGS. 4a to 4c.

First, the wafer 40, which is in the state shown in FIG. 4a, was set inthe neutral beam irradiation unit 100 of the apparatus shown in FIG. 3,and the neutral beam was irradiated in the same manner as in Example 3.The thickness of the SiO₂ mask 46 was set to 20 nm which was thinnerthan the thickness thereof shown in Example 3. By the neutral Ar beamirradiation, the reduction-resistant layer 46a was formed on the surfaceof the SiO₂ mask 46, as shown in FIG. 5a.

The wafer 40 was transferred into the magnetic microwave plasma etchingunit 101 where the Al-1%Si-0.5%Cu layer 45 thereof was etched to halfits film thickness, that is by approximately 200 nm, under the sameconditions as those in Example 3. The state of the wafer 40 at this timeis shown in FIG. 5b. Although the reduction-resistant layer 46a is shownto have been removed completely for convenience, the quantity of removalis smaller than that in Example 3, so that the reduction-resistant layer46a may not be removed completely.

The wafer 40 was restored into the neutral beam irradiation unit 100 forirradiation with the neutral Ar beam irradiation for therebyregenerating the reduction-resistant layer 46a on the surface of theSiO₂ mask 46, as shown in FIG. 5c.

The wafer 40 was again transferred into the magnetic plasma etching unit101 where the remaining portion of the Al-1%Si-0.5% Cu layer 45 and thebarrier metal layer 44 were etched for producing the Al-Cu-Simetallization layer 45a and the barrier metal pattern 44a presentingshape anisotropy.

In the present Example, the etching process of the Al-1%Si-0.5%Cu layer45 was divided into two process steps and the step of the neutral Arbeam irradiation was interposed between these steps for augmentingreduction resistance of the SiO₂ mask 46. As a result thereof, the SiO₂mask 46 served the purpose as an etching mask, despite the fact that thefilm thickness of the SiO₂ mask 46 was thinner than that in Example 3,thus favoring the subsequent planarizing step by the interlayerinsulating film. The resistance against after-corrosion was similarlyexcellent.

Meanwhile, although the number of repetition of the irradiation step andthe etching step was two in the above Example, mask selectivity may beimproved further by increasing the number of times of repetition of theirradiation and etching steps.

EXAMPLE 5

In the present Example, the Al-1%Si-0.5%Cu layer and the barrier metallayer were etched by neutral beam assisted etching.

The construction of a neutral beam assisted etching apparatus employedin the present Example is shown schematically in FIG. 6.

The neutral beam assisted etching apparatus 102 includes an ionizingchamber 50 for ionizing a feed gas supplied by means of a gas supplyduct 51 in a direction shown by arrow I, an etching chamber 53 connectedto the ionizing chamber 50 by means of a multiaperture electrode 52 andhousing a wafer stage 55 on which to set the wafer 40, and a chargedparticle removal electrode 57 mounted above the wafer 40 within theetching chamber 53 for removing residual charged particles. The insideof the etching chamber 53 is connected to a vacuum source, not shown, bymeans of a vent duct 54, whereby it is evacuated to high vacuum in thedirection shown by arrow F in FIG. 6.

A cooling conduit 6 is embedded in the wafer stage 55 and a coolingmedium is circulated from a cooing device, such as a chiller, connectedto outside, in the direction shown by arrows G₁, G₂ in FIG. 6 formaintaining the wafer 40 in the cooled state. By cooling the wafer 40 inthis manner, neutral active species (radicals) supplied from an ECRplasma supplying unit may be adsorbed on the wafer 40.

The ECR plasma supplying unit is arranged on a sidewall section of theetching chamber 53. In the ECR plasma supplying section, an etching gasis introduced into an ECR plasma generating chamber 61 via an etchinggas supply duct 62 in the direction shown by arrow H in FIG. 6. Themicrowave generated by magnetron 58 is introduced into the ECR plasmagenerating chamber 61 via a wave guide 59 and a microwave inlet window60. A plasma P₂ is generated by ECR discharge by the interaction betweenthe microwave and the magnetic field generated by a solenoid coil 63.This plasma P₂ is introduced into the etching chamber 53 via plasmainlet window 64. The plasma naturally includes ions accelerated alongthe divergent magnetic field. However, with the present apparatus, thewafer 40 is arranged in the direction normal to the incident directionof the neutral beam without being oriented along the direction of theplasma P₂. Besides, the charged particle removal electrode 57 isprovided above the wafer 40, so that a minor amount of ions may therebybe trapped. Thus only radicals having no motional directivity areallowed to travel through the electrode 57 so as to be adsorbed on thewafer surface. These adsorbed radicals are assisted by the kineticenergy of the neutral beam to progress the etching reaction.

A further typical process employing the above apparatus is explained byreferring to FIG. 4a and to FIGS. 7a and 7b. It is noted that, in FIGS.7a and 7b, parts, which are the same as those in FIGS. 4a to 4c aredepicted by the same numerals.

First, the wafer 40 which is in the state shown in FIG. 4a was set on awafer stage 55 of the above mentioned neutral beam assisted etchingdevice 102, and the wafer 40 was cooled to about -30° C. by circulatingethanol through cooling conduit 56. An etching gas was supplied frometching gas supply duct 62 under the conditions of a BCl₃ flow rate of30 SCCM, a Cl₂ flow rate of 20 SCCN and an HBr flow rate of 40 SCCM andmicrowave discharge was carried out under the conditions of a gaspressure of 1 Pa (7.5 mTorr) and a microwave power of 1 kW (13.56 MHz).From this plasma P₂, radicals such as Cl* or Br* were introduced intothe etching chamber 53 so as to be adsorbed on the cooled wafer surfaceby means of the charged particle removal electrode 57. Simultaneously, aneutral Ar beam was irradiated on the wafer 40.

In the course of this etching process, the reduction-resistant layer 46ais present at all times on the SiO₂ mask 46 because the wafer 40 isirradiated at all times with the neutral Ar beam. In this manner,selectivity becomes substantially infinite. On the other hand, thekinetic energy of the neutral Ar beam is transferred to Cl* or Br*adsorbed on the surface of the Al-1%-Si-0.5%Cu layer 45 so that reactionproducts such as SiCl_(x) or SiBr_(x) are produced. Etching proceededunder a mechanism in which the elimination of the reaction products waspromoted by the contribution of the above mentioned kinetic energy. Theetching mechanism for the barrier metal layer is similar to thatdescribed above.

In the present Example, the Al-Si-Cu metallization layer 45a havingsatisfactory shape anisotropy could be produced with highly satisfactoryresistance against after-corrosion.

It is to be noted that the present invention is not limited to the seventypical Examples and may be suitably modified with respect to theetching gas composition, etching conditions, wafer construction orconstruction of the units or the apparatus.

What is claimed is:
 1. A dry etching method for etching a layer of analuminum-based material with the aid of a pattern constituted by a layerof a silicon oxide material as a mask, comprising steps ofirradiatingsaid wafer with a neutral beam for forming a reduction-resistant layeron at least the surface of said pattern, and subsequently etching saidlayer of the aluminum-based material.
 2. A dry etching method as claimedin claim 1 wherein said neutral beam is a neutral AF beam.
 3. A dryetching method for etching a layer of an aluminum-based material withthe aid of a pattern constituted by a layer of a silicon oxide materialas a mask, comprising repeating the sequential steps ofirradiating saidwafer with a neutral beam, and partially etching said layer of thealuminum-based material along the thickness thereof.
 4. A dry etchingmethod as claimed in claim 3 wherein said neutral beam is a neutral Arbeam.
 5. A dry etching method for etching a layer of an aluminum-basedmaterial with the aid of a pattern constituted by a layer of a siliconoxide material as a mask, comprising the step ofirradiating said waferwith a neutral beam and simultaneously etching said layer of thealuminum-based material.
 6. A dry etching method as claimed in claim 5wherein said neutral beam is a neutral Ar beam.